This invention relates to internal combustion valvetrains, and similar mechanisms, and, in particular, to a cam lobe shape used in such valve trains.
Partially due to resonant frequencies and component inertias, high-speed spring-biased camshaft systems, such as the valve train of an internal combustion engine, all have a limiting speed where the excitation frequency exceeds the reaction frequency of the return spring. The excitation frequency of a high-speed engine valvetrain is determined by camshaft lobe shape characteristics and operating speed. The reaction frequency is determined by the system inertia, return spring force and natural frequencies of the spring and components. Attempts at raising the engine limit speed currently involve: 1) lowering the system inertia by using parts with lower mass and 2) increasing the return spring pressure. Either method is beneficial however current racing trends have dictated that both methods be exploited to their fullest extent, leaving no more limit speed gains possible through these common industry practices. Another industry trend in the pursuit of higher power per RPM is to quicken the opening ramp of the camshaft lobe because it has been proven that this increases power. This practice is severely limited by the necessity of performing the above methods 1 & 2 to an even further extent.
Through the speed range, a spring-biased valve train normally undergoes three modes of operation. At low to medium RPMs, the system is in controlled mode. The return spring is adequate to keep the components in contact with each other, transmitting the prescribed cam motion through the system to the valve. Approaching the limit speed, it enters the loft mode when the return spring cannot keep the components in contact with each other. In FIG. 1 (and FIGS. 2 and 4), the prescribed motion (the motion prescribed by the cam lobe) is indicated as a solid line and the actual motion of the valve is indicated as a dashed line. The valve train is compressed at 51 from acceleration of the camshaft lobe against the resistance of inertia of the components, lofted at 52, causing gaps between the components so that their motion no longer follows that prescribed by the camshaft, and collides one or more times at 53 at a period prescribed by the natural frequencies of the spring and other system parts.
As the RPMs increase further, the bounce mode is reached where the closing valve imparts collision energy into the cylinder head and this energy reacts to bounce the valve off the valve seat. The spring and component oscillation frequencies have remained constant, but because the camshaft is spinning faster, the cam lobe frequency has increased. This causes the collision of parts to occur later relative to the cam lobe. See FIG. 2. The collision at 53 has occurred so close to the bottom of the closing ramp that the energy cannot be absorbed entirely by the valve train and is transmitted to the interface of the valve and valve seat. At point 54, an elastic reaction from the collision is begining that acts to bounce the valve off the seat at 55. The loss of sealing in this mode can reduce volumetric efficiency of the engine and the increased vibrational frequencies created can often break valve springs.